Page 187 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

156 Reservoir Engineering

microlog is run with a proximity log, it is presented in track 1 with a micro-
caliper. The proximity log is presented in tracks 2 and 3 on a logarithmic scale
(Figure 5-87).
Since its introduction, the microlog (Schlumberger) has become the standard
tool for recognizing permeable zones. The theory behind it is similar to using
multiple resistivity devices. The tool consists of three electrode buttons on a
rubber pad which is pressed against the borehole wall.
In a permeable zone, mud filtrate will enter the formation leaving the clay
particles behind on the borehole wall. These clay particles may form a mudcake
up to an inch thick. The resistivity of the mudcake is less than the resistivity of
the formation saturated with mud filtrate. Two resistivity readings, the micro-
inverse and the micronormal, are taken simultaneously. The microinverse has a
depth of investigation of only an inch; therefore, it reads mostly mudcake (if

Figure 5-87. Example log showing positive separation of the microlog curves
opposite a permeable formation at 4,030 to 4,050 feet.